A system was developed to generate, measure and deliver O3 concentrations of from 0.05-10 ppm. Cultured lung fibroblasts exposed to concentrations as low as 0.05 ppm had their growth inhibited 14 plus or minus 1% over a 4 day period after exposure, and 79 plus or minus 5.4% at 4.05 ppm for a total exposure time of 30 min. The O3 concentration producing 50% growth inhibition (IC50) was 0.8 ppm with 95% confidence limits of 0.2-3.3 ppm. Ozone was not toxic for cells with an overlay of ca. 1.5 mm of balanced salt solution.